Engine oil composition

ABSTRACT

Engine oil compositions comprise (A) a lubricating base oil having a kinematic viscosity at 100° C. of 2 to 6 mm 2 /S, a viscosity index of 120 or more, and a total aromatics content of 15 percent by mass or less and (B) a polymethacrylate-based viscosity index improver in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm 2 /s. Disclosed are also engine oil compositions which comprise (A) a lubricating base oil having a kinematic viscosity at 100° C. of 2 to 6 mm 2 /S, a viscosity index of 120 or more, and a total aromatics content of 15 percent by mass or less; (B) a polymethacrylate-based viscosity index improver in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm 2 /s; and (C) molybdenumdithiocarbamate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to engine oil compositions, and more particularly to engine oil compositions which provide excellent fuel efficiency and viscosity at low temperatures and are less in evaporation loss.

[0003] 2. Description of the Prior Art

[0004] The fuel consumption reduction of automobile engines implemented since the oil crisis is still one of the important issues from the view point of resource- and environment-protection. The fuel consumption reduction of automobiles has been put into practice by reducing the body weight of an automobile, improving combustion efficiency, and reducing the occurrence of friction in an engine. The reduction of fiction in engines has been implemented by improving the movable valve structures, reducing the number of piston rings, smoothing the abrasive surfaces of sliding parts, and using fuel efficient engine oils.

[0005] Among these measures for reducing fuel consumption, the use of such fuel efficient engine oils has become general in the market because of their excellent balance of cost and performances. The engine oils are blended with effective additives such as friction modifiers. However, in order to make friction modifiers exhibit their performances sufficiently, it is important to carefully select a base oil and formulate the other engine oil additives.

[0006] Japanese Patent Laid-Open Publication No. 8-302378 discloses an engine oil composition which comprises a specific base oil, an alkaline earth metal salicylate-based detergent, zinc dialkyldithiophosphate, a polybutenylsuccinimide-based ashless dispersant, a phenol-based ashless oxidation inhibitor, a molybdenumdithiocarbamate-based friction modifier, a viscosity index improves, in a specific amount, respectively.

[0007] Reducing the viscosity of an engine oil is considered to be one of the measure to provide an engine oil with good fuel efficiency. However, no particular examination or research has not sufficiently been done on a base oil or additives for a low viscosity engine oil.

[0008] An object of the present invention is to provide an engine oil composition which is reduced more in viscosity than conventional fuel efficient engine oils and provide excellent fuel efficiency and viscosity characteristics at low temperatures with less evaporation loss by blending suitable additives.

[0009] As a result of an extensive research and development, it was found that an engine oil composition which is reduced more in viscosity than conventional fuel efficient engine oils and provide excellent fuel efficiency and viscosity characteristics at low temperatures and is less in evaporation loss can be obtained by blending a specific base oil with an specific amount of a polymethacrylate-based viscosity index improver.

BRIEF SUMMARY OF THE PRESENT INVENTION

[0010] According to the present invention, there is provided an engine oil composition which comprises (A) a lubricant base oil having a kinematic viscosity at 100° C. of 2 to 6 mm²/S, a viscosity index of 120 or more and a total aromatic content of 15 percent by mass or less and (B) a polymethacrylate-based viscosity index improver blended in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm²/s.

[0011] Furthermore, according to the present invention, there is provided an engine oil composition which comprises (A) a lubricant base oil having a kinematic viscosity at 100° C. of 2 to 6 mm²/S, a viscosity index of 120 or more and a total aromatics content of 15 percent by mass or less; (B) a polymethacrylate-based viscosity index improver blended in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm²/s; and (C) molybdenumdithiocarbamate.

[0012] The polymethacrylate-based viscosity index improver have preferably have an weight-average molecular weight of 180,000 or more.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0013] The present invention is described below in more detail.

[0014] A lubricating base oil referred to as Component (A) in an engine oil composition according to the present invention has a kinematic viscosity at 100° C. of which upper limit is 6 mm²/s, preferably 5 mm²/s and lower limit is 2 mm²/s, preferably 3 mm²/s. Lubricant base oils in excess of the upper limit would lead to increased fluid resistance resulting in increased loss caused by wear occurring at engine parts to be lubricated, while those of less than the lower limit would lead to insufficient oil-film formation, resulting in less lubricity and increased evaporation loss.

[0015] Component (A) has necessarily a viscosity index of 120 or more. Such a viscosity index value is contributive to the production of an engine oil composition having excellent low-temperature viscosity characteristics. Base oils having a viscosity index of less than 120 would lead to a necessity to bring it down lower viscosity, resulting in the increased evaporation loss and viscosity of the resulting engine oil.

[0016] The upper limit of aromatic content of Component (A) is 15 percent by mass, preferably 10 percent by mass, and most preferably 5 percent by mass. Base oils in excess of the upper limit would fail to achieve synergistic effects with each additive to be used in the present invention. No particular limitation is imposed on the lower limit of aromatic content. However, Component (A) has preferably a total aromatic content of 2 percent by mass or more because that having a total aromatic content of less than 2 percent by mass would not possibly exhibit solubility to various additives.

[0017] The term “total aromatic content” used herein denotes an aromatic fraction content measured in accordance with ASTM D2549. Incorporated by the aromatic fraction are generally alkylbenzenes, alkylnaphthalenes, anthracene, phenanthrene, alkylated products thereof, compounds in which 4 or more benzene rings are condensed, and compounds having hetero-aromatics, such as pyridines, quinolines, phenols, and naphthols.

[0018] Eligible base oils for the present invention are mineral lubricating oils, synthetic lubricating oils, and mixtures of two or more of these oils mixed in a suitable ratio.

[0019] For instance, the base oils are exemplified by mineral lubricating oils, mixtures of mineral lubricating oils and non-aromatic containing synthetic lubricating oils, and mixtures of aromatic-containing synthetic lubricating oils and non-aromatics containing synthetic lubricating oils.

[0020] The term “mineral lubricating oil” used herein denotes not only a single mineral lubricating oil but also a mixture of two or more mineral lubricating oils. Therefore, when using two more mineral lubricating oils as the base oil, there may be used not only a mixture of mineral lubricating oils each having a total aromatic content of 15 percent by mass or less but also a mixture of a mineral lubricating oil having a total aromatic content of 15 percent by mass and a mineral lubricating oil having a total aromatic content exceeding 15 percent by mass as long as the resulting base oil has a total aromatic content of 15 percent by mass or less.

[0021] Furthermore, when using a mixture of a mineral lubricating oil and a non-aromatic containing synthetic lubricating oil, there may be used a mineral lubricating oil having a total aromatic content exceeding 15 percent by mass as long as the resulting base oil has a total aromatic content of 15 percent by mass or less.

[0022] Specific examples of the mineral lubricating oil are those obtained by subjecting a lubricant fraction obtained by vacuum-distilling an atmospheric residue derived from the atmospheric distillation of crude oil to one or more refining processes such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, and hydrorefining.

[0023] Specific examples of the aromatic-containing synthetic lubricating oil are alkylnaphthalenes and alkylbenzenes.

[0024] Specific examples of the non-aromatic containing synthetic lubricating oil are polybutens and hydrides thereof; poly-α-olefins such as 1-octene oligomer and 1-decene oligomer, and hydrides thereof; diesters such as ditridecylglutarate, di-2-ethylhexyladipate, disodecyladipate, and di-2-ethylhexylcebacate; polyolesters such as trimethylolpropanecaprylate, trimethylolpropanepelargonate, pentaerythritol-2-ehylhexyanoate, and pentaerythritolpelargonate; and mixtures thereof.

[0025] Each of these lubricating oil exhibits its peculiar viscosity-temperature characteristics, i.e., viscosity index. As long as a lubricating oil used as a base oil of the present invention has a viscosity index of 120 or more, even though a lubricating base selected from the above has a viscosity index of less than 120, it may be used in combination with those having a viscosity index of 120 or more.

[0026] Component (B) of an engine oil composition according to the present invention is a polymethacrylate-based viscosity index improver blended in such an amount that the resulting composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm²/s. The kinematic viscosity at 100° C. of the resulting composition in excess of 9.3 mm²/S would not provide sufficient fuel efficiency, while that of less than 4.0 mm²/s would improve fuel efficiency caused by the reduced viscosity of the composition and viscosity at low temperatures but fail to have sufficient lubricity as an engine oil.

[0027] The combination of a base oil with such a polymethacrylate-based viscosity index improver in an engine oil composition according to the present invention results in enhanced viscosity index improving effects, less thickening effects, and excellent pour point reduction effects. The polymethacrylate-based viscosity index improver is indispensable in an engine oil composition according to the present invention in order to provide it with excellent low temperature characteristics.

[0028] Whereas, when using known polyoleinfin copolymer-based viscosity index improver, the same effects as the present invention can not be achieved.

[0029] The polymethacrylate-based viscosity index improvers which may be used in the present invention are any type of non-dispersion type or dispersion type polymethacrylate compounds which are used as viscosity index improvers for a lubricating oil.

[0030] The non-dispersion type polymethacrylate-based viscosity index improver may be a polymer of a compound represented by the formula

[0031] In formula (1), R¹ is a straight or branched alkyl group such as methyl, ethyl, propyl, butyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, and octadecyl groups.

[0032] Specific examples of the dispersion type polymethacrylate-based viscosity index improver are copolymers obtained by copolymerizing one or more monomers selected from compounds represented by formula (1) with one or more nitrogen-containing monomers selected from compounds represented by formulae (2) and (3)

[0033] In formulae (2) and (3), R² and R⁴ are each independently hydrogen or methyl. R³ is a straight or branched alkylene group having 1 to 18 carbon atoms, such as ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, dodecylene, tridecylene, tetradecylene, pentadecylene, hexadecylene, heptadecylene, and octadecylene groups. e is an integer of 0 or 1. X1 and X2 are each independently an amino- or heterocyclic-residue having 1 or 2 nitrogen and 0 to 2 oxygen. Specific examples of X1 and X2 are dimethylamino, diethylamino, dipropylamino, dibutylamino, anilino, toluidino, xylidino, acetylamino, benzoilamino, morpholino, pyrolyl, pyridyl, methylpydidyl, pyrolidinyl, piperidinyl, quinonyl, pyrrolidonyl, pyrrolidono, imidazolino, and pyrazino groups.

[0034] Specific examples of the nitrogen-containing monomer represented by formula (2) or (3) are dimethylaminomethylmethacrylate, diethylaminomethylmethacrylate, dimethylaminoethylmethacrylate, diethylaminoethylmethacrylate, 2-methyl-5-vinylpyridine, morpholinomethylmethacrylate, morpholinoethylmethacrylate, N-vinylpyrrolidone, and mixtures thereof.

[0035] Regardless of how much weight-average molecular weight the polymethacrylate-based viscosity index improver has, it can improve the low temperature viscosity characteristics. However, the lower limit of the weight-average molecular weight of the polymethacrylate-based viscosity index improver, which is effective in improving the performance of an engine oil, is preferably 180,000, more preferably 190,000. Polymethacrylate-based viscosity index improvers having a weight-average molecular weight of 180,000 or more can decrease the amount of other viscosity index improvers to be added so as to further improve low temperature viscosity, not only leading to an advantage in terms of cost but also an improvement in shear stability such that the initial performances of the resulting engine oil can be maintained. No particular limitation is imposed on the upper limit. When consideration is given to an easy treatment of the composition, it is preferably 500,000 or less and more preferably 400,000 or less.

[0036] As described above, an engine oil composition according to the present invention contains the polymethacrylate-based viscosity index improver in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm²/s. As long as the kinematic viscosity at 100° C. of an engine oil composition is within this range, the content of a polymethacrylate-based viscosity index improver may be arbitrary selected. However, the content is preferably from 0.5 to 10 percent by mass based on the total weight of the composition.

[0037] In order to further enhance fuel efficiency, an engine oil composition according to the present invention may be blended with molybdenumdithiocarbamate represented by formula (4) or mixtures thereof

[0038] In formula (4), R⁵, R⁶, R⁷, and R⁸ may be the same or different and are each independently an alkyl or alkylaryl group having 2 to 18 carbon atoms. Y¹, Y², Y³, and Y⁴ are each independently sulfur or oxygen. The alkyl group includes primary, secondary, and tertiary alkyl groups which may be straight or branched. Specific examples of the alkyl group are ethyl, propyl, butyl, pentyl, hexyl, octyl, decyl, dodecyl, and tridecyl groups. Specific examples of the molybdenumdithiocarbamate are molybdenumdiethyldithiocarbamate sulfide, molybudenumdipropyldithiocarbamate sulfide, molybdenumdibutyldithiocarbamate sulfide, molybdenumdipentyldithiocarbamate sulfide, molybdenumdihexyldithiocarbamate sulfide, molybdenumdioctyldithiocarbamate sulfide, molybdenumdidecyldithiocarbamate sulfide, molybdenumdidodecyldithiocarbamate sulfide, molybdenumditridecyldithiocarbamate sulfide, molybdenumdi(butylphenyl)dithiocarbamate sulfide, molybdenumdi(nonylphenyl)dithiocarbamate sulfide, oxymolybdenumdiethyldithiocarbamate sulfide, oxymolybdenumdipropyldithiocarbamate sulfide, oxymolybdenumdibutyldithiocarbamate sulfide, oxymolybdenumdipentyldithiocarbamate sulfide, oxymolybdenumdihexyldithiocarbamate sulfide, oxymolybdenumdioctyldithiocarbamate sulfide, oxymolybdenumdidecyldithiocarbamate sulfide, oxymolybdenumdidodecyldithiocarbamate sulfide, oxymolybdenumditridecyldithiocarbamate sulfide, oxymolybdenumdi(butylphenyl)dithiocarbamate sulfide, and oxymolybdenumdi(nonylphenyl)dithiocarbamate sulfide. Mixtures of these compounds may also be used.

[0039] The upper limit molybdenum content is 0.15 percent by mass, preferably 0.10 percent by mass, in terms of molybdenum concentration, based on the total mass of the composition. The content in excess of the upper limit would cause the formation of sludge when the engine oil is deteriorated. No particular limitation is imposed on the lower limit molybdenum content. However, the lower limit is preferably 0.02 percent by mass, more preferably 0.04 percent by mass in terms of molybdenum concentration, based on the total mass of the composition in order to obtain a sufficient friction reduction effect.

[0040] As described above, an engine oil composition according to the present invention excels in fuel efficiency and low temperature viscosity and is less in evaporation loss by blending a specific base oil with a polymethacrylate-based viscosity index improver so as to obtain a specific viscosity. Furthermore, the use of a polymethacrylate-based viscosity index improver having a weight average molecular weight of 180,000 or more can further improve fuel efficiency and low temperature viscosity. Higher level of fuel efficiency can be provided in an engine oil by adding thereto molybdenumdithiocarbamate.

[0041] For the purpose of enhancing these various performances and other various performances required for an engine oil composition, known engine oil additives may be used singlely or in combination.

[0042] Examples of such known additives which may be used in the present invention are alkaline earth metal-based detergents, ashless dispersants, corrosion inhibitors, ashless oxidation inhibitors, friction modifiers other than molybdenumdithiocarbamate, corrosion inhibitors, demulsifying agents, metal deactivators, and antifoamers.

[0043] Eligible alkaline earth metal-based detergents are alkaline earth metal compounds which are added in a lubricating oil. Specific examples of such a detergent are one or more metallic detergents selected from alkaline earth metal sulfonates, alkaline earth metal phenates, and alkaline earth metal salicylates.

[0044] Preferred alkaline earth metal sulfonates are alkaline earth metal salts, preferably magnesium salt and/or calcium salt of an alkyl aromatic sulfonic acid obtained by sulfonating an alkyl aromatic compound having a molecular weigh of 300 to 1,500, preferably 400 to 700. The latter is more preferred.

[0045] The above-mentioned alkyl aromatic sulfonic acid may be a petroleum sulfonic acid and a synthetic sulfonic acid.

[0046] The petroleum sulfonic acid may be mahogany acid obtained by sulfonating the alkyl aromatic compound contained in the lubricant fraction of mineral oil or by-produced upon the production of white oil. The synthetic sulfonic acid may be those obtained by sulfonating alkyl benzene having a straight or branched alkyl group, which may be by-produced from a plant for producing alkyl benzene used as material of detergents, or sulfonating dinonylnaphthalene. Although not restricted, there may be used fuming sulfuric acid and sulfuric acid as a sulfonating agent.

[0047] The alkaline earth metal phenate may be an alkaline earth metal salt, preferably magnesium salt and/or calcium salt of alkylphenol, alkylphenolsulfide, or a product resulting from Mannich reaction of the alkylphenol. Specific examples are those represented by the formulae

[0048] wherein R⁹, R¹⁰, R¹¹, R¹², R¹³, and R¹⁴ may be the same or different and are each independently a straight or branched alkyl group having 4 to 30, preferably 6 to 18 carbon atoms, M¹, M², and M³ are each independently an alkaline earth metal, preferably calcium and/or magnesium, and x is an integer of 1 or 2.

[0049] Specific examples of R⁹, R¹⁰, R¹¹, R¹², R¹³, and R¹⁴ are butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl, heneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl, and triacontyl groups, all of which may be straight or branched and primary, secondary or tertiary alkyl groups.

[0050] The alkaline earth metal salicylate may be an alkaline earth metal salt, preferably magnesium salt and/or calcium salt of an alkyl salicylate. Specific examples are those represented by the formula

[0051] wherein R¹⁵ is a straight or branched alkyl group having 4 to 30, preferably 6 to 18 carbon atoms, and M⁴ is an alkaline earth metal, preferably calcium and/or magnesium.

[0052] Specific examples of R¹⁵ are butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl, heneicosyl, docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl, and triacontyl groups, all of which may be straight or branched and primary, secondary or tertiary alkyl groups.

[0053] Moreover, the alkaline earth metal sulfonate, alkaline earth metal phenate and alkaline earth metal salicylate may be a neutral alkaline earth metal sulfonate, alkaline earth metal phenate and alkaline earth metal salicylate obtained by directly reacting a compound such as the above-mentioned alkyl aromatic sulfonic acid, alkylphenol, alkylphenol sulfide and the Mannich reaction product thereof, and alkyl salicylic acid with an alkaline earth metal oxide or hydroxide of magnesium and/or calcium, or obtained by converting the compound into an alkali metal salt such as sodium salt or potassium salt and then substituting the alkali metal salt with an alkaline earth metal salt. The alkaline earth metal sulfonate, alkaline earth metal phenate or alkaline earth metal salicylate may also be a basic alkaline earth metal sulfonate, alkaline earth metal phenate or alkaline earth metal salicylate obtained by heating a neutral alkaline earth metal sulfonate, alkaline earth metal phenate or alkaline earth metal salicylate in water containing an excess amount of an alkaline earth metal salt or an alkaline earth metal base; or an overbased alkaline earth metal sulfonate, alkaline earth metal phenate or alkaline earth metal salicylate obtained by reacting a neutral alkaline earth metal sulfonate, alkaline earth metal phenate or alkaline earth metal salicylate with the carbonic acid salt or boric acid salt of an alkaline earth metal in the presence of carbon dioxide.

[0054] In the present invention, there may be used the above-described neutral alkaline earth metal salt, basic alkaline earth metal salt, overbased alkaline earth metal salt, and mixtures thereof.

[0055] Commercially available metallic detergents are usually diluted with a light lubricating base oil. It is preferred to use metallic detergents containing metal in an amount of 1.0 to 20 percent by mass, preferably 2.0 to 16 percent by mass.

[0056] No particular limitation is imposed on the total base number of the alkaline earth metal detergent used in the present invention. However, preferred metallic detergents are those having a total base number of 30 to 400 mgKOH/g, preferably 150 to 300 mgKOH/g. The term “total base number” used herein denotes a total base number measured by the perchloric acid potentiometric titration method in accordance with section 7 of JIS K2501 “Petroleum products and lubricants-Determination of neutralization number”

[0057] Although not restricted, the content of the alkaline earth metal detergent is within the range of 1.0 to 10.0 percent by mass, preferably 1.0 to 8.0 percent by mass, more preferably 1.5 to 5.0 percent by mass, based on the total mass of the composition.

[0058] Preferred ashless dispersant are any type of polybutenylsuccinimides used in a lubricating oil. Specific examples of such dispersants are mono-type imides represented by formula (9), bis-type imides represented by formula (10), and those modified with organic acid or boric acid

[0059] In formulae (9) and (10), R¹⁶, R¹⁷, and R¹⁸ are each independently a polybutenyl group having a number-average molecular weight of 900 to 3,500, preferably 1,000 to 3,000, and c is an integer of 2 to 5.

[0060] No particular limitation is imposed on a method for producing the polybutenylsuccinimides. For instance, the polybutenylsuccinimides may be obtained by reacting polybutenylsuccinate resulting from the reaction of a polybutene or chlorinated polybutene having a number-average molecular weight of 900 to 3,500, with maleic anhydride. Specific examples of the polyamine are diethyltriamine, triethylenetetraamine, tetraethylenepentamine, and pentaethylenehexamine.

[0061] The upper limit content of the polybutenylsuccinimide is 0.20 percent by mass, preferably 0.10 percent by mass, in terms of nitrogen concentration, based on the total mass of the composition. Contents in excess of the upper limit would adversely affect rubber-made sealing materials of an engine. No particular limitation is imposed on the lower limit content of the polybutenylsuccinimide. However, the lower limit is preferably 0.05 percent by mass, more preferably 0.06 percent by mass, in terms of nitrogen concentration, based on the total mass of the composition such that a more sufficient fuel efficiency can be achieved.

[0062] Alternatively, an engine oil composition may be blended with one or more of other ashless dispersants such as a long chain polyalkylamine, and an amide of a long chain fatty acid and a polyamine or with those in combination with the above-described polybutenylsuccinimide ashless dispersant.

[0063] Wear inhibitors used in the present invention may be one or more dialkyldithio zinc phosphate selected from compounds represented by formula (11)

[0064] In formula (11), R¹⁹, R²⁰, R²¹, and R²² are each independently a primary alkyl group having 2 to 18, preferably 4 to 12 carbon atoms or a secondary alkyl group having 3 to 18, preferably 3 to 10 carbon atoms.

[0065] The primary alkyl group having 2 to 18 carbon atoms are those represented by the formula

R²³—CH₂—  (12).

[0066] In formula (12), R²³ is a straight or branched alkyl group having 1 to 17, preferably 3 to 11 carbon atoms. Specific examples of R²³ are straight or branched alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, and heptadecyl groups.

[0067] The secondary alkyl group having 3 to 18 carbon atoms are those represented by the formula

[0068] In formula (13), R²⁴ and R²⁵ are each independently a straight or branched alkyl group having 1 to 16, preferably 1 to 8 carbon atoms to be selected such that the total carbon number of R²⁴ and R²⁵ is 2 to 17, preferably 2 to 9 carbon atoms. Specific examples of R²⁴ and R²⁵ are straight or branched alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, and hexadecyl groups.

[0069] The upper limit content of the dialkyldithio zinc phosphate is 0.10 percent by mass, preferably 0.09 percent by mass, on an elementary basis, based on the total mass of the composition. The content in excess of the upper limit would accelerate the poisoning of a ternary catalyst adversely affecting exhaust gas. No particular limitation is imposed on the lower limit content of the dialkyldithio zinc phosphate. In order to maintain the friction coefficient after the deterioration of an engine oil, lower i.e., to maintain fuel efficiency longer, the lower limit is preferably 0.04 percent by mass, more preferably 0.06 percent by mass, on an elementary basis, based on the total mass of the composition.

[0070] An engine oil composition may be blended with one or more of other friction modifies such as organic phosphates, fatty acids, fatty acid esters, aliphatic alcohols, or with those in combination with the above-described dialkyldithio zinc phosphates.

[0071] Preferred ashless oxidation inhibitors are phenolic ashless oxidation inhibitors used as oxidation inhibitors for a lubricating oil. Specific examples of the phenolic ashless oxidation inhibitors are 4,4′-methylenebis(2,6-di-tert-butylphenol), 4,4′-bis(2,6-di-tert-butylphenol), 4,4′-bis(2-methyl-6-tert-butylphenol), 2,2′-methylenebis(4-ethyl-6-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-tert-butylphenol), 4,4′-butylidenebis(3-methyl-6-tert-butylphenol), 4,4′-isopropylidenebis(2,6-di-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-nonylphenol), 2,2′-isobutylidenebis(4,6-dimethylphenol), 2,2′-methylenebis(4-methyl-6-cyclohexylphenol), 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, 2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-butyl-4(N,N′-dimethylamino-p-cresol, 2,6-di-tert-butyl-4(N,N′-dimethylaminomethylphenol), 4,4′-thiobis(2-methyl-6-tert-butylphenol), 4,4′-thiobis(3-methyl-6-tert-butylphenol), 2,2′-thiobis(4-methyl-6-tert-butylphenol), bis(3-methyl-4-hydroxy-5-tert-butylbenzyl)sulfide, bis(3,5-di-tert-butyl-4-hydroxybenzyl)sulfide, 2,2′-thio-diethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tridecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, pentaerythrityl-tetraquis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and mixtures thereof.

[0072] An engine oil composition may be blended with one or more of the above-described ashless dispersants or with one or more of amine-based ashless dispersants such as phenyl-α-nephtylamine, alkylphenyl-α-nephtylamine, and dialkyldiphenylamine. Alternatively, the above-described phenolic ashless dispersants may be sued in combination with the amine-based ashless dispersants.

[0073] The upper limit content of the above-described ashless oxidation inhibitors is 3.0 percent by mass, preferably 2.0 percent by mass. A content in excess of the upper limit would fail to achieve oxidation inhibition that balances the amount. No particular limitation is imposed on the lower limit content. However, the lower limit content of preferably 0.1 percent by mass, more preferably 0.3 percent by mass is contributive to reduce the friction coefficient of an engine oil after being deteriorated.

[0074] An engine oil composition according to the present invention may be blended with friction modifiers other than the above-described molybdenumdithiocarbamates. Such friction modifiers may be molybdenumdithiophosphate, molybdenum disulfide, long-chain aliphatic amines, long-chain fatty acids, long-chain fatty acid esters, long-chain aliphatic alcohols.

[0075] Additives other than those of the above-described which may be used in the present invention are corrosion inhibitors such as petroleum sulfonates, alkylbenzene sulfonates, dinonylnaphthalene sulfonates, alkenylsuccinates, and polyalcohol esters; demulsifying agent, typical examples of which are polyalkylene glycol-based non-ionic surfactants such as polyoxyethylenealkyl ether, polyoxyethylenealkylphneyl ether, and polyoxyethylenealkylnaphthyl ether; metal diactivators such as imidazoline, pyrimidine derivatives, alkylthiadiazole, mercaptobenzothizole, benzotriazole and derivatives thereof, 1,3,4-thiadiazolepolysulfide, 1,3,4-thiadizolyl-2,5-bisdialkyldithiocarbamte, 2-(alkyldithio)benzoimidazole, and β-(o-carboxybenzylthio)propionnitrile; and antifoamers such as slicone, fluorosilicone, and fluoroalkyl ether.

[0076] When adding these additive to an engine oil composition according to the present invention, the corrosion inhibitors and demulsifying agents are each added in an amount of 0.1 to 15 percent by mass, the antifoamers are added in an amount of 0.0005 to 1 percent by mass, and the metal deactivators are added in an amount of 0.005 to 1 percent by mass, based on the total mass of the composition.

[0077] An engine oil composition according to the present invention may be used preferably in motorcycle engines, automobile engines, diesel engines for land use, and marine diesel engines.

[0078] The invention will be further described by way of the following examples which are provided for illustrative purposes only. The performances of engine oils used in inventive examples and comparative examples were evaluated by the following performance evaluating tests.

[0079] (1) Engine motoring test

[0080] The friction torque of the whole of an engine was measured by driving at 1,500 rpm, at an oil temperature of 80° C., and at a water temperature of 80° C. In general, an engine oil has better fuel efficiency with the smaller value which indicates the smaller friction loss at each parts of the engine.

[0081] (2) NOACK evaporation test (ASTM D 5880)

[0082] The evaporation loss of each of the engine oils was measured after being heated at a temperature of 250° C. and under a constant pressure for one hour. An engine oil with the smaller value is less consumed during actual running.

[0083] (3) CCS viscosity (ASTM D 5293)

[0084] This test evaluates the cranking performance of each of the engine oils. Engine oils with the smaller value has better low temperature viscosity characteristics.

INVENTIVE EXAMPLES 1-3

[0085] Table 1 shows the above performance evaluation test results of the engine oils of Inventive Examples 1-3. Each of the engine oils was formulated so as to have the same kinematic viscosity at 100° C. and high temperature high shear viscosity at 150° C. It is apparent from the results in Table 1 that the engine oils of Inventive Examples 1-3 had an excellent fuel efficiency, less evaporation loss, and an excellent low temperature viscosity. It was also apparent that these oils exhibited more excellent performances when being blended with a polymethacrylate-based viscosity index improver with a weight-average molecular weight of 250,000 than when being blended with that of a weight-average molecular weight of 150,000. Furthermore, it was apparent that the engine oils blend with molybdenumdithiocarbamate exhibited an excellent fuel efficiency.

Comparative Examples 1-3

[0086] Table 1 also shows the above performance evaluation test results of the engine oils of Comparative Examples 1-3. The engine oil of Comparative Example 1 with the base oil having a viscosity index of 100 was inferior in fuel efficiency, evaporation loss, and low temperature viscosity. The engine oil containing an olefin copolymer-based viscosity index improver (Comparative Examples 2) was inferior in fuel efficiency, evaporation loss, and low temperature viscosity. The engine oil of Comparative Example 3 with a kinematic viscosity of 9.3 or more was inferior in fuel efficiency even though being blended with molybdenumdithiocarbamate. TABLE 1 Inventive Inventive Inventive Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 1 Example 2 Example 3 Base oil I¹⁾ 85.9 85.9 84.3 79.7 82.3 Mass % Base oil II²⁾ 4.2 mass % Base oil III³⁾ 85.9 mass % Viscosity index 4.0 improver I⁴⁾ mass % Viscosity index 4.0 4.0 4.0 6.0 improver II⁵⁾ mass % Viscosity index 6.0 improver III⁶⁾ mass % MoDTC⁷⁾ 1.6 1.6 mass % Additive 10.1 10.1 10.1 10.1 10.1 10.1 Package⁸⁾ mass % Kinematic 8.25 8.33 8.32 8.39 8.24 9.45 viscosity (100° C.) mm²/s High 2.62 2.63 2.61 2.62 2.61 2.75 temperature high shear Viscosity (150° C.) mPa•s Engine (◯) (◯) (⊚) (x) (x) (x) motoring 19.6 19.4 18.1 20.2 20.2 20.1 friction torque Test N•m NOACK (◯) (◯) (◯) (x) (x) (◯) Evaporation 14 14 14 22 17 14 Mass % CCS viscosity (◯) (◯) (◯) (x) (x) (◯) (−25° C.) mPa•s 3250 3200 3270 4630 4960 3300

[0087] 1) hydrocracking mineral oil: 4.2 mm²/s of kinematic viscosity at 100° C., 3.1 mass % of total aromatics content, 125 of viscosity index

[0088] 2) hydrocracking mineral oil: 2.6 mm²/s of kinematic viscosity at 100° C., 2.1 mass % of total aromatics content, 104 of viscosity index

[0089] 3) solvent-refined mineral oil: 4.5 mm²/s of kinematic viscosity at 100° C., 25.3 mass % of total aromatics content, 100 of viscosity index

[0090] 4) Polymethacrylate-based viscosity index improver: 150,000 of weight-average molecular weight

[0091] 5) Polymethacrylate-based viscosity index improver: 250,000 of weight-average molecular weight

[0092] 6) Olefin copolymer-based viscosity index improver: 250,000 of weight-average molecular weight

[0093] 7) Molybdenumdithiocarbamate represented by the formula

[0094] wherein R is an alkyl group having 8 or 13 carbon atoms,

[0095] Y is oxygen or sulfur,

[0096] 4.8 mass % of molybdenum concentration

[0097] 8) Additive mixtures containing calcium sulfonate, calcium salicylate, dialkyldithio zinc phospahte, succinimide-based ashless dispersant, phenol-based oxidation inhibitor, antifoamer, and corrosion inhibitor

[0098] As described above, the present invention can provide an engine oil composition which excels in fuel efficiency and low temperature characteristics and encounters less evaporation loss. 

What is claimed is:
 1. An engine oil composition which comprises (A) a lubricating base oil having a kinematic viscosity at 100° C. of 2 to 6 mm²/S, a viscosity index of 120 or more, and a total aromatic content of 15 percent by mass or less and (B) a polymethacrylate-based viscosity index improver blended in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm²/s.
 2. An engine oil composition which comprises (A) a lubricating base oil having a kinematic viscosity at 100° C. of 2 to 6 mm²/S, a viscosity index of 120 or more, and a total aromatics content of 15 percent by mass or less; (B) a polymethacrylate-based viscosity index improver blended in such an amount that the composition has a kinematic viscosity at 100° C. of 4.0 to 9.3 mm²/s; and (C) molybdenumdithiocarbamate.
 3. The engine oil composition according to claim 1 wherein said polymethacrylate-based viscosity index improver has a weight-average molecular weight of 180,000 or more.
 4. The engine oil composition according to claim 2 wherein said polymethacrylate-based viscosity index improver has a weight-average molecular weight of 180,000 or more.
 5. The engine oil composition according to claim 1 wherein said total aromatics content is 5 percent by mass or less.
 6. The engine oil composition according to claim 2 wherein said total aromatics content is 5 percent by mass or less.
 7. The engine oil composition according to claim 1 wherein said polymethacrylate-based viscosity index improver is a polymer of a compound represented by the formula

 wherein R¹ is a straight or branched alkyl group having 1 to 18 carbon atoms.
 8. The engine oil composition according to claim 2 wherein said polymethacrylate-based viscosity index improver is a polymer of a compound represented by the formula

 wherein R¹ is a straight or branched alkyl group having 1 to 18 carbon atoms.
 9. The engine oil composition according to claim 1 wherein said polymethacrylate-based viscosity index improver is a copolymer obtained by copolymerizing one or more monomers selected form the group consisting of compounds represented by formula (1) with one or more nitrogen-containing monomers selected from the group consisting of compounds represented by the formulae

 wherein R² and R⁴ are each independently hydrogen or methyl, R³ is a straight or branched alkylene group having 2 to 18 carbon atoms, e is an integer of 0 or 1, and X1 and X2 are each independently an amine residue or heterocyclic ring having 1 or 2 nitrogen and 0 to 2 oxygen.
 10. The engine oil composition according to claim 2 wherein said polymethacrylate-based viscosity index improver is a copolymer obtained by copolymerizing one or more monomers selected form the group consisting of compounds represented by formula (1) with one or more nitrogen-containing monomers selected from the group consisting of compounds represented by the formulae

 wherein R² and R⁴ are each independently hydrogen or methyl, R³ is a straight or branched alkylene group having 2 to 18 carbon atoms, e is an integer of 0 or 1, and X1 and X2 are each independently an amine residue or heterocyclic ring having 1 or 2 nitrogen and 0 to 2 oxygen.
 11. The engine oil composition according to claim 2 wherein said molybdenumdithiocarbamate is contained in an amount of 0.02 to 0.15 percent by mass in terms of molybdenum concentration, based on the total mass of the composition.
 12. The engine oil composition according to claim 2 wherein said molybdenumdithiocarbamate is represented by the formula

 wherein R⁵, R⁶, R⁷, and R⁸ are the same or different and are each independently an alkyl or alkylaryl having 2 to 18 carbon atoms, and Y¹, Y², Y³, and Y⁴ are each independently selected from the group consisting of sulfur and oxygen.
 13. The engine oil composition according to claim 1 which further comprises additives selected from the group consisting of alkaline earth metal detergents, ashless dispersants, wear inhibitors, ashless oxidation inhibitors, friction modifiers other than molybdenumdithiocarbamate, corrosion inhibitors, demulsifying agents, metal deactivators, and antifoamers.
 14. The engine oil composition according to claim 2 which further comprises additives selected from the group consisting of alkaline earth metal detergents, ashless dispersants, wear inhibitors, ashless oxidation inhibitors, friction modifiers other than molybdenumdithiocarbamate, corrosion inhibitors, demulsifying agents, metal deactivators, and antifoamers. 